1. Field of the Invention
The invention relates to an electric power steering system including a steering operation assist motor.
2. Description of Related Art
The torque of a steering operation assist motor (will hereinafter be simply referred to as “electric motor”) used in an electric power steering system is controlled by supplying currents to stator coils of the electric motor according to the vehicle speed and the steering operation torque. The electric motor is powered by a power source, such as a storage battery. However, because the power source has its powering limit, the power supplied to the electric motor is limited based on the powering limit of the power source. More specifically, the current supplied from the power source is limited at or below a certain level according to the powering limit of the power source.
Japanese Patent Application Publication No. 2005-271640 describes a method that is used in a vehicle that has an idle-stop function. According to this method, in order to reduce the load on a power source when a steering wheel is manually operated while an engine is in an idle-stop state, the drive force of an electric motor is limited to be equal to or smaller than a predetermined level when the engine is in the idle-stop state. This makes it easier to restart the engine. Further, Japanese Patent Application Publication No. 2010-173417 describes a method in which an upper limit value of current to be supplied to an electric motor is reduced as time passes from when the amount by which the steering wheel has been manually operated during an idle-stop state of the engine (i.e., the steering operation torque or the steering operation speed) exceeds a set steering operation amount.
According to the method described in Japanese Patent Application Publication No. 2005-271640, the rotation speed of the electric motor is gradually reduced from when the engine is automatically stopped. Therefore, for example, if the steering wheel is operated when a certain amount of time has elapsed since an automatic stop of the engine, a driver may have a feeling of strangeness that the steering wheel has suddenly become heavy to turn, which is undesirable. According to the method described in Japanese Patent Application Publication No. 2010-173417, once a state where the steering operation torque is larger than a set level is detected, the steering operation assist is limited as time passes from the detection. Therefore, for example, when the driver operates the steering wheel again some time after operating the steering wheel and then taking his or her hands off the steering wheel, because the electric motor current is largely limited at this time, the driver may have, as in the case of the foregoing method, a feeling of strangeness that the steering wheel has suddenly become heavy to turn, which is also undesirable.